Ah, Piers Anthony. Brilliant world-builder, creative storyteller, has more issues than a high school full of teenagers.
I was another huge fan of his back in my teenaged years, but have lost interest in him as I went into my twenties. It definately helps if you read his stuff when a teenager, as his approach to sexuality tends to go from "fascinating" to "creepy" somewhere along the line. I also second that his series' get steadily worse at some point. I thought the first trilogy of the Apprentice Adept to be better than the second (thought that might be because the second trilogy uses way to much of Piers' warped concept of honor.) And the seventh book was just weird.
As for Xanth, I recommend reading them in order, mostly because that's another series that largely degrades with time. I found the first fifteen or so to be enjoyable, but it starts to degrade around Demons Don't Dream. The puns start to become excessive, and they overuse the weird "moon" concept and the Demon Xanth "Save the Universe" plots.